The present invention relates to fiber optic networks, and more particularly to fiber optic wavelength division multiplexers and demultiplexers.
The transmission capacity of fiber-optic communication systems has increased significantly by use of wavelength division multiplexing (WDM) techniques. In a WDM communication system, multiple channels, where each channel is differentiated by using a unique wavelength of light, carry modulated optical signals in a single optical fiber between a transmitter and a receiver. The transmitter uses an optical multiplexer to combine multiple channels into the fiber for transmission, and the receiver uses an optical demultiplexer to separate the optical channels for detection. FIG. 1 illustrates a typical optical demultiplexer (demux) 120 containing a single input port 110 and multiple output ports 130-1 through 130-N, where each optical channel from the input port is mapped to a unique output port in sequential order (channel 1 will exit from port 130-1, channel 2 from port 130-2, etc.). Optical multiplexers are simply demultiplexers operated in the reverse direction, where a specific wavelength has to be supplied to the correct input port to emerge at the output port as a multiplexed signal.
It is expected that in the foreseeable future, communication systems will evolve to communication networks consisting of multiple access nodes, each containing a WDM transmitter and/or receiver, that are interconnected in some prescribed fashion (e.g., ring or bus) or arbitrarily (e.g., mesh). Information flow between two access nodes will be carried on an available optical wavelength that is assigned by a protocol according to network availability. The transmitting node will have to employ a bank of lasers at different wavelengths as available sources, all connected properly to the multiplexer""s ports, utilizing only a small fraction of the lasers at any given time for communication. This is clearly an expensive solution, as most of the hardware is lying idle. Alternatively, wavelength tunable lasers can be used. However, tunable lasers cannot be connected directly to the optical multiplexer, as the multiplexer""s input ports can only accept the correct wavelength to function properly.
FIG. 2 illustrates a possible solution, consisting of a switching matrix 220 added to the node, whose role is to route the tunable lasers""s signals 210-1 through 210-M to the correct input ports 230-1 through 230-N of multiplexer 240. This added hardware is again costly. It is clear that the receiving node will also have to address the same issues for the demultiplexing and detection task.
In accordance with the present invention, a programmable optical multiplexer/demultiplexer can establish a reconfigurable connection between any two ports from the multiple device ports, independently for each optical wavelength that is inserted by the input ports.
In one embodiment of the present invention, a programmable demultiplexer is arranged to receive an input signal containing components at N different wavelengths from an optical input port, and distribute the input signal components among K output ports. The input signal is collimated by a particular lens in a microlens array, which lens is aligned to the input port. The microlens array contains K additional lenses that are aligned to the K output ports. The resultant collimated beam originating from the input port is then made incident on a diffraction grating, which angularly disperses the composite optical signal according to wavelength, thereby forming N separate beams having different wavelengths and distinct propagation angles. Each of the N separate beams propagates to a single lens that is arranged to collect all the beams and provide, for each wavelength, a converging beam focused onto a particular micro-mirror in an array containing N micro-mirrors. Each mirror in the array is individually controlled to reflect the incident beam (representing a corresponding wavelength) in a desired direction, such that it will (a) re-enter the lens, (b) be collimated by the lens and redirected to a different location on the diffraction grating, and (c) be eventually coupled from the diffraction grating through a particular lens in the micro-lens array to a desired output port (the particular micro-lens is aligned to the desired output port). Generally, the number of output ports K and optical wavelength components N are independent. The demultiplexer can be designed to operate in the regime where K=N, so that each wavelength component can be assigned to any output port. The invention can also be operated in a mode where K less than N, in which case more than one wavelength is applied to an output port, or in a mode where K greater than N, in which case one or more output ports are not used. In any event, the present invention enables assignment of any wavelength to any output port.
The embodiment just described can be operated in the xe2x80x9creversexe2x80x9d direction, in order to act as a programmable multiplexer, rather than as a demultiplexer. In the multiplexer arrangement, K input signals each containing one or more different wavelengths, are received from a plurality of K optical input ports and must be combined and made available at a single output port. The K input signals cumulatively contain a total of N different wavelengths, or, stated differently, any particular wavelength component can exist at only one of the K input ports, or contention will occur. Each input signal is collimated by a respective lens in a microlens array that contains K+1 lenses. One lens is aligned with the output port, while the remaining lenses are aligned each to a corresponding input port. The resultant collimated beam originating from each input port is then made incident on a diffraction grating, which diffracts the optical signal as a function of its wavelength. The diffraction grating is arranged such that all of the separate beams, which have different wavelengths and therefore distinct propagation angles, propagate to a single lens that collects all the beams and provides, for each wavelength, a converging beam focused onto a particular micro-mirror in an array. Each mirror in the array is individually controlled to reflect the incident beam (representing a corresponding wavelength) in the desired direction, such that it will (a) re-enter the lens, (b) be collimated by the lens and redirected to a single location on the diffraction grating, and (c) be eventually coupled from the diffraction grating to the output port through the particular lens in the micro-lens array that is aligned with the output port. Here again, in general, the number of input ports K and optical wavelength components N are independent. The multiplexer can be designed to operate in the regime where K=N, so that each wavelength component can originate at any input port. The invention can also be operated in a mode where K less than N, in which case more than one wavelength is applied to an input port, or in a mode where K greater than N, in which case one or more input ports are not used. In any event, the present invention enables multiplexing (combining) of all input wavelengths originating at the K input ports to the output port.